The present invention relates to a clad sheet comprising a metal substrate and a metal cladding layer (between which a metal intermediate layer may be provided), and a process for producing such a sheet.
More specifically, this invention relates to a novel clad sheet in which a cladding material is cold-cladding over an entire surface, or locally on the required area of, a substrate comprising a single or laminated sheet of metal (or alloy) and, more particularly, to a clad sheet of high quality and improved integrity, which allows high-efficiency cold cladding by rolling of a continuous (or strip-like) cladding material.
The cladding materials for electronical parts now available include laminated, striped clad sheets comprising Fe-Ni substrate (40 to 55% Ni-Fe)-Al sheet (substrate-cladding layer; this arrangement shall hereinafter hold), Fe-Ni substrate (40 to 55% Ni-Fe)-Ag solder sheet, Fe-Ni substrate (40 to 55% Ni-Fe)-Ag sheet, Kovar (trade name) alloy sheet-Ag solder sheet and the like, or full-surface clad sheets comprising Fe-Ni substrate (40 to 55% Ni-Fe)-Al sheet, Kovar alloy sheet (25 to 50% Ni, 10 to 20% Ni-Fe)-Ag solder sheet and the like. (If not otherwise specified, "%" denotes by weight in this application.)
For instance, Al striped clad sheets are generally produced by the following process. A metal substrate strip (or ribbon) of a 42% Ni-Fe alloy is annealed in a reducing atmosphere to clean out its surface, following which buffing is applied on a surface portion of the substrate to be cold-clad for further cleaning. At least one Al stripe of a given pattern is superposed upon that substrate for cold-cladding by rolling. Thereupon, in an as-clad state or after at least one cold-rolling has been applied, the resulting product is subjected to diffusion annealing at a temperature of 600.degree. C. or lower to complete bonding of the Al stripe to the metal substrate. Furthermore, to adjust the size and shape of the clad sheets, the thus obtained product is subjected to at least one cold-rolling and is then heat-treated to apply tension and increase elongation to the clad stripe(s) in order to remove internal stress remaining ununiformly in the widthwise direction of the substrate due to striped cladding or stress remaining in the edge portions thereof after stamping-out, e.g., to make lead frames.
With mechanical polishing such as wire buffing, however, there is a fear that polishing may be extended from the required surface area of the substrate to be predetermined to be clad to other surface areas, for instance, to the entire surface thereof, resulting in cracking of the polished surface, occurrence and deposition of scaly metal powders and residence of dust. This may lead to a problem that, in bonding of the cladding layer, metal powders, said dust or gases are caught up on its surface to be clad, which may cause surface blistering.
Referring also to the so-called striped clad sheet having one or plural stripes of cladding layer clad on its surface, in particular, cracking tends to produce fine metal powders during cold-cladding by rolling or the subsequent cold-rolling, which are in turn attributable to deposition of said fine metal powders to the clad sheet, leading to a problem of deteriorations in the surface quality of the clad sheet, etc. In addition, the conventional production process needs a number of steps and heat treatment which incur an increase in the production cost, and offers problems in connection with defects occurring on the cladding material during diffusion annealing, surface-quality deteriorations.